Fluorescence imaging is employed for imaging tissues. The fluorescence imager detects fluorescence light emitted from a fluorescence agent that is excited by appropriate illumination. For example, a fluorescent microscope is an imaging system for imaging internal blood flow, visible through the skin, by promoting fluorescence radiation from a fluorescent dye added to the flowing blood. The microscope can be employed, for example, during surgery for visualizing the blood flow, and for evaluating tissue perfusion and vessel patency.
The excitation illumination should be strong (i.e., of high intensity) because the fluorescence signal is often fairly weak, especially when employing IndoCyanine Green (ICG) as the fluorescence agent. One approach to create such an excitation light source is to use a laser diode. A laser diode allows relatively high power to be concentrated in a narrow wavelength region. This is advantageous for exciting fluorescent agents without interfering with the fluorescence image (i.e., as the excitation radiation can be easily filtered out in the camera).
It is noted that the fluence (i.e., output distribution) of a laser diode is Gaussian, with the center of the beam having significantly higher energy than the edges of the beam. Put another way, the fluence of the laser diode beam is non-uniform. This presents a significant problem for fluorescence imaging since fluorescence intensity is generally proportional to excitation light. Any light source that is not uniform will create artificial differences in fluorescence intensity due to non-uniformity of the excitation light. These artificial differences may be misinterpreted as physiological effects by the user of the system viewing the image. Such a misinterpretation may have very significant clinical consequences. Additionally, if image pixel intensity measurements (of any kind) are used, a non-uniform source will produce incorrect measurement results.
When designing laser-based illumination systems, one design requirement is to provide systems that are skin-safe and eye-safe. That is, the output of the illumination system should be within the safety limits for light incident on human skin and on human eyes.
A diffuser that vibrates along its plane is known in the art. Reference is now made to Datasheet: LSR-3000 Series for Laser Speckle Reducer LSR-3000 Series, published on Mar. 10, 2013 at: http://www.optotune.com/images/products/Optotune%20LSR-000%20Series.pdf. LSR Speckle reducer is basically a diffuser that is moved (i.e., vibrated). Speckle noise from a laser-based system is reduced by dynamically diffusing the laser beam. The diffuser is bonded to a thin elastic membrane, which includes four independent electro-active polymer electrodes that induce a circular oscillation of the diffuser in X and Y directions. The oscillation frequency is set to the measured resonant frequency of the LSR speckle reducer during production. However, both voltage and frequency of the electro-active polymer can be controlled.
Semiconductor diode lasers are electrically pumped semiconductor lasers in which the active medium is formed by a p-n junction of a semiconductor diode. Semiconductor diode lasers include several configurations, such as edge-emitter laser diodes and Vertical-Cavity Surface-Emitting Diode Lasers (VCSEL). Edge-emitter laser diodes are made up of bars diced from the wafers on which the diode layers are grown. The high index of refraction contrast between air and the semiconductor material at the side facets of the diced bars act as mirrors. Thus, the light oscillates parallel to the layers and escapes side-ways.
In a VCSEL, the active layer is sandwiched between two highly reflective mirrors (also referred to as distributed Bragg reflectors) composed of several layers of alternating high and low refractive index. The light oscillates perpendicular to the layers and escapes through the top (or bottom) of the device. A VCSEL array is an X-Y array of thousands of laser sources packed into a rectangular illuminator (e.g., 2.8 millimeter×2.8 millimeter). Each individual illuminator in the array is fairly low power (e.g., a few milliwatts). However, taken together the thousands of illuminators make up a powerful illuminator array. VCSEL array products are known in the art, for example, a “6 W CW 808 nm VCSEL Array” by Princeton Optronics (http://www.princetonoptronics.com/products/pdfs/PCW-CS6-6-W0808%20revB-0514.pdf). It is noted that each ray (produced by a single illuminator) in the array has a non-uniform fluence (e.g., Gaussian shaped fluence). Therefore, while the fluence of a beam formed by the multitude of rays is more uniform than that of any of the rays, it still resembles a pin cushion, and cannot be considered as smoothly uniform.
Reference is now made to U.S. Pat. No. 8,016,449 issued to Liu et al., and entitled “Surface Light Emitting Apparatus Emitting Laser Light”. This publication relates to a surface light emitting apparatus, which can be employed, for example, as a backlight for a screen. The apparatus includes a laser light source, and an optical element. The surface of the optical element has optical power, and it converts the intensity distribution of the laser beam emitted by the laser light source into a uniform intensity distribution.
Reference is now made to International Patent Application Publication No. WO2011/059383 to Ivarsson et al., and entitled “Optical Sensor System Based on Attenuated Total Reflection and Method of Sensing”. This publication relates to an optical sensor system employing surface plasmon resonance (SPR). The system includes a laser light source, an SPR detector and a distribution device. The distribution device is located between the laser light source and the SPR detector. The laser light source can emit IR radiation. The distribution device distributes the laser beam emitted by the laser light source and converts it into a uniform intensity distribution beam. The system determines the dip in the detected spectrum intensity profile (i.e., the location of the low point in the intensity profile).
Reference is now made to an article by Reinhard Voelkel et al., and entitled “Laser Beam Homogenizing: Limitations and Constraints”, published at SPIE_7102_19, Optical Design Conf., Laser Beam Homogenizing, Glasgow 2008. This publication relates to laser homogenizing systems. For example, this publication describes a laser homogenizing system employing an array of lenses for converting the intensity distribution of a laser beam into a uniform intensity distribution.